New type of quantum computer

Physicists have managed to quantum entangle more than a dozen photons in an efficient and predefined way. The achievement lays the groundwork for a new kind of quantum computer.

The phenomena of the quantum world, so exotic that they often seem impossible from the perspective of the common everyday world, can nevertheless be harnessed for practical applications with current technology or that is already being developed. For example, quantum entanglement: a quantum connection between particles that binds them together in strange ways, even if they are separated by colossal distances. It can be used, for example, in a quantum computer, a machine that, unlike a conventional computer, can perform numerous mathematical operations simultaneously. However, to use a quantum computer profitably, a large number of quantum-entangled particles must work together. Only in this way will a sufficiently large number of quantum bits or qubits be achieved.

Photons, the particles of light, are particularly well suited to serve as qubits because they are naturally robust and easy to manipulate.

The team of Philip Thomas, from the Max Planck Institute for Quantum Optics in Germany, has now managed to take an important step so that photons can be used in a truly practical way in technological applications such as quantum computing: for the first time, the team generated up to 14 quantum entangled photons in a predefined way and with high efficiency.

The key to achieving this high-precision, on-demand quantum entanglement was that the researchers used a single atom to emit the photons and quantum entangle them in a very specific way. To do this, the researchers placed a rubidium atom in the center of an optical cavity, a kind of echo chamber for electromagnetic waves. With a laser light of a certain frequency, it was possible to act precisely on the state of the atom. Using an additional control pulse, the researchers also specifically caused the emission of a photon entangled with the quantum state of the atom.

They repeated this process several times and in a previously determined manner. Meanwhile, the atom was manipulated in a certain way (in technical jargon, it was rotated). In this way, it was possible to create a chain of up to 14 light particles that were quantum entangled with each other by the rotations of the atom and that were brought to the desired state. As far as Thomas and his colleagues know, the set of 14 interconnected light particles is the largest number of quantum-entangled photons ever generated in a laboratory.

A single rubidium atom is trapped in an optical resonator made up of two highly reflective mirrors. Repeated excitation of the atom causes several quantum-entangled single photons to be emitted successively. (Image: MPQ)

But it is not just the number of entangled photons that is a big step towards the development of powerful quantum computers: the way they are generated is also very different from conventional methods. By those methods, the quantum-entangled photons settle essentially randomly and uncontrollably. This also limits the number of particles that can be grouped together in a collective state. Instead, with the new system, each control pulse actually delivers a photon with the desired properties. And basically any number of entangled photons can be generated.

In addition, the new method is much more efficient than the previous ones.

All of this can make it easier to build quantum computers that are powerful and robust enough.

Thomas and his colleagues present the technical details of their new system in the academic journal Nature, under the title “Efficient generation of entangled multiphoton graph states from a single atom”. (Font: NCYT by Amazings)